Corrosive attack from concentrations of caustic chemicals has been known to occur in nuclear steam generator heat transfer tubing at and near the tube sheet. The attack is aggravated by the presence of residual stresses in the tubing that may be induced during manufacture by rolling of the tube into the tube sheet. The highest stresses usually occur at the transition from the rolling termination to the unrolled tube which is usually near the surface of the tube sheet facing the incoming tubing. Presently, operating chemistry is the major one defense against such caustic attack. Another defense is thermal treatment of the tubing as a last phase of manufacture at the tube mill to increase its resistance to chemical attack. It is also known to sleeve the tube in and adjacent to the tube sheet in order to provide two distinct barriers to corrosion.
Unfortunately, with some prior art sleeve designs, the connection between the tube and the tube sheet does not adequately defend against corrosion and in some cases, the designs utilized are not structurally adequate to withstand thermal and mechanical stresses.
Droin, in U.S. Pat. No. 4,071,083, discloses a tubular heat exchanger where the tubes are provided with short ferrules of austenitic stainless steel which are welded end to end with the tubes. Such a configuration provides only a single layer corrosion barrier and introduces a potentially troublesome tube weld inside the tube sheet.
Chapman, in U.S. Pat. No. 2,966,340, discloses a steam generator which uses corrosion resistant sleeves positioned over the ends of the tubes and connected to the tubing by brazing. The sleeve ends of the tubes are then expanded into a bore in a tube sheet and welded in place. Chapman is typical of the fillet type of joint wherein an abrupt transition occurs between the reinforced and nonreinforced portions of the tube which, as further explained below, tends to result in corrosion and structural problems.
Young, in U.S. Pat. No. 2,368,391, discloses thick-walled sleeves which are brazed to the ends of thin-walled copper tubing at the ends where the tubes are inserted into the tube sheet of the heat exchanger. The sleeves are then welded to the tube sheet thus preventing the "burning" of the thin-walled tube during bonding to the relatively thick tube sheet.
Rosenblad, in U.S. Pat. No. 2,349,792, discloses a method for replacing tubes in a tube sheet of a heat exchanger. The resulting tube-to-tube sheet connection has an abrupt transition at the connection which gives rise to the above-mentioned structural and corrosion problems.
Jacobus, in U.S. Pat. Nos. 2,209,974 and 2,209,975, discloses a tubular heat exchange apparatus having a tube-to-tube sheet connection for preventing any loosening of the tubes during high-pressure, high-temperature operation. The connection comprises a swaged tube end having ribs and a ferrule sleeve telescoped over the swaged tube end, the ferrule sleeve fitting tightly over the ribs so as to be spaced from the tube. The ferrule is then welded to the tube and swaged and expanded into a hole in the tube sheet. The ferrule and the tube end are designed to be able to expand differentially so that the tubes can expand against and contract from the ferrules.
Brown, in U.S. Pat. No. 1,856,618, discloses an air or gas heat exchanger having air or gas carrying finned tubes which are easily removable from the tube sheets. A sleeve is positioned in a hole in the tube sheet which is of sufficient diameter to allow the fins to be pulled through the hole. When the sheet is removed, the tube can be withdrawn through the hole.
The prior art fails to teach a steam generator having a connection between a length of stock steam generator tubing, a sleeve member, and a tube sheet where the connection and a transition formed between the stock tubing and the sleeve are designed to avoid the introduction of corrosion sites or structural weaknesses.